1. Field of the Invention
The present invention relates to a vacuum drying machine with multiple tables for drying industrial hides and similar products.
Drying machines of this kind generally comprise a fixed supporting frame and a plurality of horizontal work tables which are controllably vertically movable with respect to the fixed supporting structure. Each of said tables has, on its upper face, a supporting and heating plate for supporting and heating the hides and, on its lower face, a closure lid which can sealingly engage with the supporting plate of the underlying table. The coupling between each lid and the related underlying table defines an evaporation chamber in which the hides being dried are placed.
2. Description of the Related Art
In known drying machines, see for example WO-A-8705944, said tables are commonly moved vertically by pairs of single- or double-action hydraulic cylinders arranged in positions adjacent to the uprights of the supporting frame.
In said machines, the jackets of the hydraulic cylinders are fixed to the base of the supporting frame, while the ends of their piston rods are connected to the respective tables by means of appropriate connecting elements. Said known movement systems have several advantages, but they are not free from some disadvantages, the main one of which is that very long cylinders must be used in order to achieve the required work strokes, with jackets which are sometimes so long that they have to be accommodated in corresponding cavities provided in the ground. Said machines, once installed, consequently cannot be easily transferred without entailing high costs.
In other known kinds of drying machines, an attempt has been made to obviate the above-described disadvantage by providing every table with a pair of double-action linear cylinders and movable jackets, in which said jackets are connected to the lateral ends of the tables, while the piston rods are fixed to the upper ends of the respective uprights of the supporting frame. With this solution, the cylinders thus hang from the respective piston rods and it is therefore not necessary to provide appropriate cavities in the ground to accommodate them. However, even in this solution there is the disadvantage of a considerable vertical extension of the machine, with evident limitations as to the maximum number of usable tables which, for practical reasons, cannot normally be more than four. This limitation severely affects the productive requirements of the drying cycle. In order to optimize the use of personnel in the drying process, it is in fact convenient to increase the time required to place hides on all of the work tables so that this time is not shorter than the time over which a vacuum is maintained in the evaporation chamber. In known machines which have a reduced number of tables, the hide placing operation requires considerably less time than the hide evaporation operation, so that there is an inefficient use of personnel, with a consequent loss in the economy of the production cycle.
Another aspect related to the limitation of the maximum number of tables of a machine is constituted by the final quality of the hides at the end of the drying process, which depends to a large extent on the vacuum drying times. In fact it is known that the prolongation of said times allows for a reduction in other parameters of the process, such as for example the temperature or the degree of vacuum, which negatively affect the structure of the treated product if they are excessively high. In the drying machines of the prior art the treated product's quality cannot therefore be improved without negatively affecting the economy of the production cycle.
A further disadvantage of conventional drying machines resides in the large number of parts which constitute the movement devices, which entail high costs for the production and assembly of the machine and give rise to considerable maintenance expenses for the necessary large number of spare parts which must be kept in stock.
A further disadvantage of said machines is constituted by their great bulk and by the difficult transportability of the parts which compose the supporting structure. This is mostly due to the movement systems commonly employed, which normally extend along the entire height of the machine and must therefore be integrated in the uprights of the supporting structure.